Brake systems for vehicles (e.g., aircraft, automobiles, etc.) are well known in the art. Most brake systems include a brake actuator for exerting pressure on brake material. The brake material in turn exerts a braking torque on the element to be braked (e.g., the wheel of the vehicle). The brake actuator may be hydraulic or electromechanical, for example. By selectively activating the brake actuator, a desired amount of braking torque, i.e., desired torque, or force, may be applied to the element to be braked.
In the past, torque feedback has been considered desirable in braking applications to compensate for various effects. For example, brake systems for vehicles have included a torque controller that utilizes the measured torque applied to the wheel to compensate for brake fade (due to thermal effects) and grabby brakes (common with carbon brakes). A torque sensor would measure the torque applied to the wheel, and the output of the torque sensor was fed back to the torque controller. The torque controller would modulate the brake actuator to apply the desired pressure to a brake to achieve the desired torque applied to the wheel.
Various problems arose, however, as a result of the use of torque feedback. For example, due to sensor noise and physical properties of torque, the output of the torque sensor, i.e., the measured torque signal, was not valid at or near zero wheel speed. To account for this, the torque feedback was disabled below a predefined wheel speed and the brake system would revert to open loop control. This “low speed cutout” of the torque feedback to the torque controller would naturally have to occur at a speed at which the output of the torque sensor was still valid. Since torque sensors typically are valid only to a predefined lower speed limit, the low speed cutout was required to occur at a speed greater than the lower speed limit. Thus, the limitations of the torque sensor precluded torque compensation at low wheel speeds.
Additional problems occur in torque controllers that employ a Proportional/Integral/Derivative (PID) controller. Measured torque is compared to the desired torque and a resulting torque error signal drives the PID controller to a desired pressure. However, the transfer function from pressure, i.e., clamp force, to torque occurs faster than the brake actuator generating the clamp force can respond. Additionally, the PID controller must be robust enough to compensate for changes in brake friction that normally occur during a stop.
In view of the aforementioned problems associated with torque controllers using torque feedback, there is a strong need in the art for a torque controller that calculates the pressure to be applied by the brake actuator based on measured torque in a time period in which the brake actuator can respond. Additionally, there is a need for a torque controller that can compute the pressure to be applied by the brake actuator at or near zero wheel speed. In addition, there is a strong need for such a torque controller which is not computationally intensive and which does not require multiple sensors, etc.